1. Field of the Invention
The present invention relates to the sputtering of thin films onto a substrate and more particularly, to an apparatus and a method for clamping the substrate so as to substantially eliminate excessive gap sizes between the substrate and the backplane which degrade thermal transfer to the substrate.
2. Background of the Invention
An integrated circuit (IC) is manufactured by a process which utilizes planar technology. Generally, this process includes subjecting a substrate, such as a silicon or a gallium arsenide wafer, to a sputtering process in which a thin layer or film of metal is deposited on the substrate.
It is essential that the wafer be maintained at a predetermined temperature which is distributed uniformly on the wafer during the sputtering process. In order to achieve this, the wafer is placed on a thermal backplane which serves to maintain the wafer at the predetermined temperature through conduction. However, due to surface imperfections and other irregularities, substantial areas of the wafer and the backplane are not in actual physical contact with each other, thus hindering uniform temperature distribution on the wafer. A thermally conductive gas is then provided between the backplane and the wafer in order to transfer heat between areas of the backplane and wafer which are not in contact with each other to thus ensure uniform temperature distribution on the wafer.
The gas creates pressure against the wafer which moves the wafer away from the backplane, thus creating a gap between the wafer and the backplane. It is desirable that the size of gap be minimized in order to ensure uniform temperature distribution. In particular, it has been found that a gap size not exceeding approximately 0.002 inches is preferable. Gap sizes in excess of 0.002 inches undesirably result in a degradation of thermal transfer and in uniformity of temperature distribution on the wafer. Further, such gap sizes result in excess gas leakage.
During the sputtering process, the wafer is held by a clamping mechanism which provides a clamping force for restraining the wafer and maintaining a minimal gap size between the wafer and backplane. Referring to FIG. 1, a cross sectional view of one type of clamping mechanism 10 is shown. The clamping mechanism 10 is used for clamping a wafer 12 which is fabricated from a resilient material and which includes an upper 14 and lower 16 surface. In particular, the clamping mechanism 10 includes a clamp ring 18 having a contact surface 20 which contacts edges of the upper surface 14 of the wafer 12 and serves to clamp the wafer 12.
The clamping mechanism 10 is used in conjunction with a backplane 22 having a convex shaped backplane surface 24. In use, the backplane surface 24 is displaced against the lower surface 16, thus urging the wafer 12 against the contact surface 20. This restrains the wafer 12 and causes the wafer 12 to conform to the convex shape of backplane surface 24. The backplane 22 includes a central passageway 26 which extends through the backplane 22 toward the backplane surface 24. Conducting gas is fed through the central passageway 26 (shown by arrow) and is discharged against the lower surface 16 of the wafer 12 so as to create pressure against the lower surface 16 of the wafer 12, thus urging the wafer 12 against the contact surface 20 of the clamp ring 18. Essentially, the clamping force for restraining the wafer 12 and maintaining a gap size 28 not exceeding 0.002 inches between the backplane surface 24 and the lower surface 16 substantially depends upon the mechanical properties and dimensions of the wafer 12, curvature of backplane surface 24 and the pressure of the conducting gas against the wafer 12. Typically, for an eight-inch diameter silicon wafer, a total clamping force in the range of approximately 5 to 20 pounds is desirable.
The clamping mechanism 10 further includes springs 30 each of which are secured between a fixed member 32 and the clamp ring 18. The springs 30 are affixed such that they become extended and thus biased to urge the clamp ring 18 and ultimately the wafer 12 against the backplane surface 24 and withstand the pressure of the conducting gas to thus restrain the clamp ring 18 and wafer 12. In addition, the springs 30 are sized so as to provide the desired clamping force suitable for urging the wafer 12 against the backplane surface 24 and such that the gap size 28 does not exceed 0.002 inches.
In order to minimize the gap size 28, the contact surface 20 of the clamp ring 18 and the backplane surface 24 are precisely machined. Further, it is essential that the machined surfaces and the overall shape of the backplane 22 and the clamp ring 18 remain substantially stable and distortion free during the sputtering process. As such, both the backplane 22 and clamp ring 18 are fabricated so that they are relatively massive in size in order to reduce distortions due to temperature and other factors. However, the wafer 12, backplane 22 and clamping mechanism 10 are subjected to very severe environmental conditions during the sputtering process which includes frequent temperature cycling over very large temperature ranges such as, for example, between room temperature and approximately 600.degree. C. This temperature cycling is sufficient to frequently cause distortion of the clamp ring 18 and backplane 22 notwithstanding their relatively massive size and results in a loss of precision fit which undesirably increases the gap size 28 to greater than 0.002 inches and causes other undesirable effects. Further, the physical location of the clamp ring 18 results in the deposition of undesirable excess materials on the clamp ring 18 as a result of the sputtering process. This further exacerbates the problem of distortion and associated increase in the gap size 28.
Referring to FIGS. 2a and 2b, an alternate type of clamping mechanism 34 is shown. In particular, FIG. 2a is a cross sectional side view of the alternate clamping mechanism 34 and FIG. 2b is a full top view of the alternate clamping mechanism along line 2b--2b of FIG. 2a. It is noted that the alternate clamping mechanism 34 is also disclosed in U.S. Pat. No. 4,909,695 which issued to Hurwitt, et al. and is assigned to Materials Research Corporation, the assignee herein. In the alternate clamping mechanism 34, an attachment ring 36 is utilized which includes a plurality of spaced apart spring fingers 38. The spring fingers 38 are located around an outer periphery 40 of the wafer 12 and contact an associated section of the upper surface 14 of the wafer 12. Each of the spring fingers 38 are fabricated from a resilient material and serve to independently urge an associated section of the upper surface 14 against the backplane surface 24 and withstand the pressure of the conducting gas. As such, precise flatness of the attachment ring 36 or the backplane 24 is not essential since variances in flatness and other parameters are compensated for by the resiliency of each of the spring fingers 38. Consequently, relatively precise machining techniques are not needed, thus substantially reducing manufacturing costs.
However, it has been found that this type of clamping mechanism forms undesirable edge gaps between spring fingers 38. Referring to FIG. 2c, a side view along line 2c--2c of FIG. 2b is shown of an edge or spring-back gap 42 formed between successive spring fingers 38. As described previously, the wafer 12 is fabricated from a resilient material. Further, the wafer 12 is in contact with spring fingers 38 which are spaced apart from each other. As such, the wafer 12 is not supported along most of the outer periphery 40. Consequently, areas between the wafer 12 and the backplane 22 in between successive spring fingers 38 where the wafer 12 is unsupported tend to deform and form the spring-back gap 42 between each pair of spring fingers 38 along the outer periphery 40. The spring-back gap 42 allows localized leakage of the conducting gas and undesirably degrades heat transfer and uniformity of temperature distribution on the wafer 12. A further disadvantage is that a large number of parts are required to assemble the alternate clamping mechanism 34. In particular, the alternate clamping mechanism 34 includes the attachment ring 36, spring fingers 38 and the associated screws, retainers and alignment devices associated with each spring finger 38. In normal use, the alternate clamping mechanism 34 is frequently disassembled, cleaned and reassembled. As such, the large number of parts results in high maintenance costs.